ext4: fix comments in ext4_can_extents_be_merged()

Since commit a9b82415, we are allowed to merge unwritten extents,
so here these comments are wrong, remove it.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>